Root
- For other meanings of root, see Root (disambiguation).
In vascular plants, roots form the portions of a plant which generally lie below the level of the soil (cf. stem, rhizome). But this is not always characteristic for a root, since a root can also be an aerial root (i.e. entirely above the ground) or an aerating root (i.e. rises above the ground, especially above water). On the other hand, stems or leaves below ground are no exception. So, a root is better defined as a part of a plant, that bears no leaves. Therefore, it also lacks nodes.
The two major functions of roots are absorption and supply of water and inorganic nutrients to the plant, and its anchoring in the ground.
Roots will generally grow in any direction where the correct environment of air, nutrients and water exists that meets that plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks.
Remark: this article mostly discusses the roots of vascular plants. Although many algae and mosses also have some primitive water-absorption organs, the latter are hardly reminiscent of the complex structure we usually refer to by the word root.
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Types of roots
See Taproot and Fibrous root system.
A true root system consists of a primary root and secondary roots (or lateral roots).
The primary root originates in the radicle of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized:
- the taproot: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward.
- the primary root is not dominant : the whole root system is fibrous and branches in all directions.
Adventitous roots arise from the stem and not from another root. They usually occur in monocots and pteridophytes, but also in a few dicots, such as strawberry (Fragaria vesca) and white clover (Trifolium repens).
Specialized roots
- aerating roots (or pneumatophores): roots rising above the ground, especially above water such as in mangrovegenera (Avicennia, Sonneratia)
- aerial roots : roots entirely above the ground, such as in ivy (Hedera helix) or in epiphytic orchids. They function as prop roots or anchor roots.
- contractile roots : they pull bulbs or corms of monocots deeper in the soil through expanding radially and contracting longitudinally. They show a wrinkled surface.
- haustorial roots : roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and Rafflesia.
- stilt roots : these are adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil.
- storage roots : these roots are modified for storage of nutrients, such as carrots and beets
- tubiferous roots : forms rounded knobs (tuber) for food storage, such as the potato.
Root growth
At germination, roots grow downward due to gravitropism, the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as ivy), the "root" actually clings to walls and structures; this is known as thigmotropism, or response to touch.
Most plants experience growth only along the apical meristems; this is known as primary growth, which encompasses all vertical growth. On the other hand, secondary growth encompasses all lateral growth, a major component of woody plant tissues. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm, found only in woody plants.
In woody plants, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The cambium layer forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to be pushed outward and are eventually "sloughed off" (shed).
At this point, the cork cambium (noting that this process only occurs in woody plants) begins to form the periderm, consisting of protective cork cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
The vascular cambium produces new layers of secondary xylem annually. This dead tissue is responsible for most water transport through the vascular tissue (systems and roots).
Root cap
At the apex of every root there is a thimble-like cover, the root cap. It usually is not visible to the naked eye. It consists of undifferentiated soft tissue with unthickened walls, repairing the worn-out cells by forming new cells at the growing tip, or apical meristem. It is also involved in the production of mucigel, a sticky mucilage coating the new formed cells. These cells contain statoliths, starch grains which move in response to gravity and thus control root orientation.
Structure
The outside surface of a root is known as the epidermis. Recently produced epidermal cells absorb water from the surrounding environment and produce root hairs that increase the absorptive surface. Root-hairs are generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as osmosis. Water that is saline is more difficult for most plant species to absorb.
Beneath the epidermis is the cortex, which comprises the bulk of the root. Its main function is the storage of starch; its many intercellular spaces also aerate cells for respiration. The endodermis is the layer surrounding the vascular cylinder (or stele). The tightly packed cells of the endodermis contain a substance known as suberin that creates an impermeable barrier. Water can only flow in one direction: into the center of the root, rather than outward from the stele into the cortex.
The vascular cylinder contains all cells within the endodermis. The outer portion, known as the pericycle, surrounds the actual vascular tissue. In monocotyledons, the xylem and phloem cells are located in a circle around the pith, or center of a monocot root's stele, whereas in dicotyledons, the xylem cells form a central "hub" with lobes, and the phloem cells fill in the space between the lobes.
See also
- Rhizophilous - organisms which thrive in a proximity or in a symbiotic relationship with plant roots.
- Related terms: fibrous root system -- radicle -- root cap -- taproot system
References
- Brundrett M. C. (2002). Coevolution of roots and mycorrhizas of land plants. New phytologist 154(2), 275-304. (Available online: DOI (http://dx.doi.org/10.1046/j.1469-8137.2002.00397.x) | Abstract (http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/abs/) | Full text (HTML) (http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/full/) | Full text (PDF) (http://www.newphytologist.org/Brundrett.pdf))
- R. Chen, E. Rosen, P. H. Masson (1999). Gravitropism in Higher Plants. Plant Physiology 120 (2), 343-350. (Available online: Full text (HTML) (http://www.plantphysiol.org/cgi/content/full/120/2/343) | Full text (PDF) (http://www.plantphysiol.org/cgi/reprint/120/2/343.pdf)) - article about how the roots sense gravity.
- Lynn Clark (2004). Primary Root Structure and Development (http://www.eeob.iastate.edu/classes/bot404/docs/404root104.pdf) - lecture notes
- J. A. Raven, D. Edwards (2001). Roots: evolutionary origins and biogeochemical significance. Journal of Experimental Botany 52 (Suppl 1), 381-401. (Available online: Abstract (http://jxb.oupjournals.org/cgi/content/abstract/52/suppl_1/381) | Full text (HTML) (http://jxb.oupjournals.org/cgi/content/full/52/suppl_1/381) | Full text (PDF) (http://jxb.oupjournals.org/cgi/reprint/52/suppl_1/381.pdf))
External link
Introduction to Botany - University of Arkansas (http://www.ualr.edu/~botany/roots.html)
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Categories: Plant physiology